1. Field of the Invention
The present invention concerns an organic electroluminescence device.
2. Description of the Related Art
Organic electroluminescence devices are light emitting devices capable of operating at low voltage and suitable to saving of power consumption. Accordingly, they have now been used generally as light emitting devices for use in display devices and illumination devices. The organic electroluminescence device usually has a structure in which a lower electrode, an organic layer formed by laminating an organic hole transporting layer and an organic light emitting layer, and an upper electrode in this order above a substrate. Further, one of the lower electrode and the upper electrode sandwiching the organic layer is used as an anode and the other of them is used as a cathode.
A structure in which the upper electrode is formed as a transparent electrode and emission light caused in the device is taken out on the side of the upper electrode is referred to as a top emission organic electroluminescence device. There is also a structure of using a semi-transparent upper electrode thereby resonating emission light generated in the device. Further, there is also a structure of taking out light from both of the upper electrode and the lower electrode.
In a structure using the upper electrode as a transparent electrode, in a case of using the upper electrode as a cathode (that is, transparent cathode), a portion in contact with the organic layer has an electron injecting layer particularly formed of a material having high light transmittance selected from materials having low work function. However, it may be difficult to find materials of sufficiently high light transmittance among the materials having low work function.
Then, it has been proposed a first structure having an electron injecting layer of a super thin film comprising a metal material having low work function disposed to a portion on the side of a cathode in contact with an organic layer, and a transparent conductive layer comprising an Indium-Tin-Oxide (ITO), Indium-Zinc-Oxide (IZO); etc. disposed further thereon (refer to JP No. 3560375 and JP-A No. 10-162959).
Further, it has been proposed a second structure having the electron injecting layer as a mixed layer comprising a metal material having low work function and an electron transporting organic material (refer to JP-A No. 10-162959).
Further, it has been proposed a third structure having an electron injecting layer of a laminate structure. In this case, the electron injecting layer is formed by laminating a metal layer comprising a metal material having low work function, and a mixed layer comprising a metal material having low work function and an electron transporting organic material orderly from the side of the organic layer. The electron injecting layer is disposed directly on the light emission layer (refer to JP-A 2004-296410).